Man in a mix-up (3)
I believe the answer is:
ian
'man' is the definition.
(man's name)
'in a mix-up' is the wordplay.
'mix-up' indicates anagramming the letters.
'in'+'a'='ina'
'ina' anagrammed gives 'IAN'.
(Other definitions for ian that I've seen before include "Scottish man's name" , "Botham or Chappell?" , "- Botham, cricketer" , ". . . Botham, . . . Woosnam" , "- - Wallace (baritone)" .)